The present invention relates to a conversion circuit adapted to convert a small signal voltage to a current or voltage having a magnitude proportional to that of the small signal voltage, which signal voltage varies with reference to the high potential of a DC power supply source.
It is often needed in electronic circuits to produce a current or voltage proportional to a small signal voltage. A voltage-to-current or voltage-to-voltage conversion circuit is employed to achieve this purpose. FIG. 1 shows a prior art voltage-to-current converter circuit disclosed in an article entitled "AN INTEGRABLE PRECISION VOLTAGE-TO-CURRENT CONVERTER WITH BILATERAL CAPABILITY" by S. Pookaiyaudom et al, IEEE J. Solid-State Circuits, vol. SC-13, no. 3, pp. 411-413, June 1978. As shown, the circuit is comprised of a pair of constant current supply sources 1 and 2 for supplying constant current Iref currents as shown, NPN transistors Q.sub.1 -Q.sub.5, and a pair of input resistors 3 and 4. An input voltage Vin is applied between the input resistor 3 and the negative power supply terminal of a DC power source. Namely, the input voltage Vin is applied to the circuit with the negative potential of a DC power source taken as reference.
If the output current is represented by I, the base-to-emitter voltages of transistors Q.sub.1 -Q.sub.4 are represented respectively by V.sub.BE1, V.sub.BE2, V.sub.BE3 and V.sub.BE4, and the resistance values of resistors 3 and 4 are represented respectively by R.sub.3 and R.sub.4 in the circuit, the following equation will be obtained: EQU V.sub.BE1 +V.sub.BE4 +(Iref+I)R.sub.4 =V.sub.BE2 +V.sub.BE3 +Iref.multidot.R.sub.3 +Vin (1)
With V.sub.BE1 =V.sub.BE3, V.sub.BE2 =V.sub.BE4, and R.sub.3 =R.sub.4 =R, equation (1) will be rewritten as follows: EQU I.multidot.R=Vin (2)
It is therefore understood that output current I is given by Vin/R and proportional to input voltage Vin.
However, because output current I flows through transistor Q.sub.4 and resistor 4 in addition to constant current Iref of constant current supply source 2 in the prior art circuit shown in FIG. 1, unbalance is caused between collector currents of transistors Q.sub.3 and Q.sub.4 when input voltage Vin becomes large. For the purpose of preventing degradation of the accuracy of output current I because of this unbalance between collector currents, the prior art circuit needs means for preventing the unbalance between collector currents, which renders the circuit arrangement complicated.
A more important problem of prior art circuit is that the input voltage Vin cannot be applied to the circuit without taking the low potential of a DC power source as a reference. Even if the polarity of the power source and the conductivity type of transistors Q.sub.1 -Q.sub.5 employed in the prior art circuit are reversed so as to convert an input voltage, which varies with respect to the high potential of DC power source, to current or voltage, no high accuracy conversion circuit will be realized.